1. Field of the Invention
The invention relates to a testing method for a printed circuit board, more particularly to a testing method for a printed circuit board formed with conductive traces for high-frequency differential signal transmission.
2. Description of the Related Art
In order to process accurately a differential operation between positive and negative differential signals, the differential signals should be transmitted to a parallel and proximate pair of conductive traces. When the conductive traces are in close proximity, a high common-mode rejection ratio can be obtained. In addition, signal reflection can be eliminated and noise in a common-mode rejection can be effectively coupled. Therefore, in designing a printed circuit board, conductive traces are arranged in parallel and in close proximity on the printed circuit board. As such, undistorted differential signals can be maintained and electromagnetic interference can be minimized. However, to ensure product quality, conductive pads are formed on the printed circuit board for testing purposes.
FIG. 1 illustrates a conventional testing method for a punted circuit board 1. The printed circuit board 1 is formed with first and second conductive traces 11, 12 thereon. The conductive traces 11, 12 are configured for transmitting differential signals and have segments 111, 121. At a point along the length of each of the segments 111, 121 of the first and second conductive traces 11, 12, there is a bent section 13, 14 to accommodate conveniently a pair of test probes of a test equipment. Conductive pads 15, 16 are disposed and connected electrically and respectively to the bent sections 13, 14 in the segments 111, 121 of the first and second conductive traces 11, 12. However, since a uniform distance is not maintained between the segments 111, 121 of the first and second conductive traces 11, 12 as a result of the bent section 13, 14, electromagnetic field that causes electromagnetic interference can hardly be eliminated. In addition, since the common-made rejection ratio is reduced, noise that appears at the input cannot be effectively coupled. These result in distorted differential signals and inaccurate differential operation.